PVC Wire Insulation

Over about the last decade I have encountered with increasing frequency a black corrosion on the strands of copper wire.
When you strip the wire it'll be jet black instead of old copper brown.
It cannot be soldered.
It is a real nuisance when trying to repair something like an automobile headlamp socket or an extension cord that needs a new end.

It used to be somewhat unusual, then it seemed to be most frequent on particular colored wires, nowadays it's prevalent.

Three questions:

1. Might this coating be copper sulfate ?

2. Might it be caused by sulfur bearing organotin stabilizers in the PVC insulation, as metal based ones are phased out?? (##&I@#!@&^ that 9** partiple EPA )

3. What will dissolve it and leave me with shiny copper that I can solder?
I've tried MEK, alcohol, acetone, paint stripper, salt & vinegar to no avail. Scraping gets only one face of outside strands leaving me a joint that I don't trust for high current.

This is aggravating handymen worldwide, just try a search.
Sure would appreciate some education here as well as practical advice .

This is a business opportunity - bottle a cure .
It's about the only product i'd order from those annoying TV ads.

I would guess copper(II) sulfide (not sulfate, which is blue) or copper(II) oxide.

If it is sulfide, try cleaning with ammonia solution. Otherwise, I think this is going to a tough one to remove chemically. The oxide dissolves in potassium cyanide - but anything that reacts chemically with the oxide or sulfide will probably react with metallic copper as well.

After more reading, the black stuff is more likely copper sulfide(? .. chemistry is not my strength. )

EDIT oops Aleph posted while I was typing THANKS !!!

Adding just a few crystals of copper sulfate biocide from the farm supply store to my salt&vinegar solution turned the black stuff to a copper color, albeit a dull one.
Rinsed it in baking soda then applied flux and it readily took solder .

That has rescued a hundred dollar extension cord (200 ft #12) that only needed a new receptacle end.
Total expenditure:
$0.59 for receptacle
$0.19 for outlet box
had everything else on hand.

I hope this helps somebody with a car , boat or household repair.

Industry needs to know if PVC stabilizers are corroding the wires they insulate - it could cause something that's important to fail.